In this century, the shortage of fresh water may surpass the shortage of energy as a global concern for humanity, and these two challenges are inexorably linked, as explained, e.g., in the “Special Report on Water” in the 20 May 2010 issue of The Economist. Fresh water is one of the most fundamental needs of humans and other organisms; each human needs to consume a minimum of about two liters per day. The world also faces greater freshwater demands from farming and industrial processes.
The hazards posed by insufficient water supplies are particularly acute. A shortage of fresh water may lead to a variety of crises, including famine, disease, death, forced mass migration, cross-region conflict/war, and collapsed ecosystems. Despite the criticality of the need for fresh water and the profound consequences of shortages, supplies of fresh water are particularly constrained. 97.5% of the water on Earth is salty, and about 70% of the remainder is locked up as ice (mostly in ice caps and glaciers), leaving only a fraction of all water on Earth as available fresh (non-saline) water.
Moreover, the earth's water that is fresh and available is not evenly distributed. For example, heavily populated countries, such as India and China, have many regions that are subject to scarce supplies. Further still, the supply of fresh water is often seasonally inconsistent. Meanwhile, demands for fresh water are tightening across the globe. Reservoirs are drying up; aquifers are falling; rivers are dying; and glaciers and ice caps are retracting. Rising populations increase demand, as do shifts in farming and increased industrialization. Climate change poses even more threats in many regions. Consequently, the number of people facing water shortages is increasing. Naturally occurring fresh water, however, is typically confined to regional drainage basins; and transport of water is expensive and energy-intensive.
One method for obtaining fresh water from sea water is membrane distillation, wherein water from a heated saline liquid stream is allowed to vaporize through a hydrophobic microporous membrane. Membrane distillation is thermally driven, where a temperature difference across the two sides of the membrane leads to a vapor-pressure difference that causes water to evaporate from the hot side of the membrane and pass through the pores as vapor to the cold side. Additionally, membrane distillation runs at relatively low pressure, can withstand high salinity feed streams, and is potentially more resistant to fouling than other distillation approaches. Consequently, membrane distillation can be used for desalination where reverse osmosis is not a practical option. The use of thermal energy, rather than electrical energy, and the fact that membranes for membrane distillation can withstand dryout make this technology attractive for renewable power applications, as well. However, most research on membrane distillation has focused on maximizing membrane flux as opposed to minimizing energy consumption and cost [see E. K. Summers, H. A. Arafat, J. H. Lienhard V, “Energy Efficiency Comparison of Single-Stage Membrane Distillation (MD) Desalination Cycles in Different Configurations,” Desalination, 290, pp. 54-66 (2011)]; and current membrane distillation systems suffer from poor energy efficiency compared to other desalination systems.
Membrane-distillation systems can be used in many configurations, depending on how liquid is collected from the permeate (cold) side. In direct-contact membrane distillation (DCMD), the vapor is condensed on a pure water stream that contacts the other side of the membrane. In air-gap membrane distillation (AGMD), an air gap separates the membrane from a cold condensing plate which collects vapor that moves across the gap. In sweeping-gas membrane distillation (SGMD), a carrier gas is used to remove the vapor, which is condensed in a separate component. SGMD is typically used for removing volatile vapors and is typically not used in desalination.
Vacuum membrane distillation (VMD) is another variation of membrane distillation in which the driving pressure difference is increased by lowering the pressure on the vapor (cold) side of the membrane. The heat of vaporization is then recovered in an external condenser. This process has been applied to the desalination of seawater. However, energy recovery is limited by the saturation temperature of the pressure in the condenser; and maximizing flux by increasing the pressure difference between the saline feed and the condenser results in poor energy recovery.
In desalination systems, recovering the energy given off in condensation increases the thermal efficiency of the system, which is strongly correlated with low water cost. Some studies of VMD from an energy efficiency point of view have been conducted, but typically report low performance. Performance as measured by the Gained Output Ratio (GOR) is below 1 for these systems [A. Criscuoli, et al., “Evaluation of energy requirements in membrane distillation”, 47 Chemical Engineering and Processing: Process Intensification, Euromembrane 2006, 1098-1105 (2008); and X. Wang, et al., “Feasibility research of potable water production via solar-heated hollow fiber membrane distillation system, 247 Desalination 403-411 (2009)].
GOR is the ratio of the latent heat of evaporation of a unit mass of product water to the amount of energy used by a desalination system to produce that unit mass of product. The higher the GOR, the better the performance. For example, a solar still would have a GOR on the order of 1, whereas a good multi-effect distillation system may have a GOR of 12.
A low GOR arises from the fact that energy recovery is limited by the saturation temperature of the pressure in the condenser. Maximizing flux by increasing the pressure difference between the saline feed and the condenser lowers the condensation temperature in the condenser, which requires high mass flow rates of colder water to condense the additional vapor, when compared to a system with a smaller pressure difference (higher saturation temperature) and lower flux. This trade-off, however, results in poor energy recovery.